A twin study is a kind of genetic study done to determine heritability. The premise is that since identical twins (especially identical twins raised apart) have identical genotypes, differences between them are solely due to environmental factors. By examining the degree to which twins are differentiated, a study may determine the extent to which a particular trait is influenced by genes or the environment.

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Twins have been of interest to scholars since early civilization, such as the early physician Hippocrates (5th c. BCE), who attributed similar diseases in twins to shared material circumstances, and the stoic philosopher Posidonius (1rst c. BCE), who attributed such similarities to shared astrological circumstances. Later, St. Augustine of Hippo (4rth c. CE) pointed to the differences in fraternal twins, such as Jacob and Esau in the Old Testament, to disprove the central tenet of astrology, that time of birth determines personality and fate.[1]

For a group of twins in which at least one member of each pair is affected, pairwise concordance is a measure of how many of each pair will have both members affected. It can be calculated with a formula of C/C+D, in which C is the number of concordant pairs and D is the number of discordant pairs.

For example, a group of 10 twins have been pre-selected to have one affected member. During the course of the study four other previously non-affected members become affected. This gives a pairwise concordance of 4/(4+6) or 4/10 or 40%.

For a group of twins in which at least one member of each pair is affected, probandwise concordance is a measure of the proportion of twins who have the illness who have an affected twin and can be calculated with the formula of 2C/(2C+D), in which C is the number of concordant pairs and D is the number of discordant pairs.

For example, a group of 10 twins that have been pre-selected to have one affected member. During the course of the study four other previously non-affected members become affected. This gives a probandwise concordance of 8/(8+6) or 8 / 14 or 57%.

Concordance studies compare traits which are either present or absent in each twin, correlational studies compare the agreement in continuously varying traits across twins.

Fig 2. Heritability for nine psychological traits as estimated from twin studies. All sources are twins raised together (sample size shown inside bars). Identical twins (MZ twins) are twice as genetically similar as fraternal twins (DZ twins) and so heritability (h^2) is approximately twice the difference in correlation between MZ and DZ twins. Unique environmental variance (e^2) is reflected by the degree to which identical twins raised together are dissimilar, and is approximated by 1-MZ correlation. The effect of shared environment (c^2) contributes to similarity in all cases and is approximated by the DZ correlation minus the difference between MZ and DZ correlations.

Koluchova, J. (1976b) A report on the further development of twins after severe and prolonged deprivation. In: A.M. Clarke and A.D.B. Clarke (eds) Early Experience: Myth and Evidence, London: Open Books.